A new decadentate dinucleating ligand containing a pyridazine bridging group and pyridylic arms has been synthesized and characterized by analytical and spectroscopic techniques. Four new dinuclear cobalt complexes featuring this ligand have been prepared and thoroughly characterized both in the solid state (X-ray diffraction) and in solution (1D and 2D NMR spectroscopy, ESI-MS, and electrochemical techniques). The flexible but stable coordination environment provided by the ligand scaffold when coordinating Co in different oxidation states is shown to play a crucial role in the performance of the set of complexes when tested as catalysts for the photochemical hydrogen evolution reaction (HER) and chemical oxygen reduction reaction (ORR).

The synthesis of new molecular complexes of U²⁺ has been pursued to make comparisons in structure, physical properties, and reactivity with the first U²⁺ complex, [K(2.2.2-cryptand)][Cp′₃U], 1 (Cp′=C₅H₄SiMe₃). Reduction of Cp′′₃U [Cp′′=C₅H₃(SiMe₃)₂] with KC₈ in the presence of 2.2.2-cryptand or 18-crown-6 generates [K(2.2.2-cryptand)][Cp′′₃U], 2-K(crypt), or [K(18-crown-6)(THF)₂][Cp′′₃U], 2-K(18c6), respectively. The UV/Vis spectra of 2-K and 1 are similar, and they are much more intense than those of U³⁺ analogues. Variable temperature magnetic susceptibility data for 1 and 2-K(crypt) reveal lower room temperature χ_MT values relative to the experimental values for the 5f³ U³⁺ precursors. Stability studies monitored by UV/Vis spectroscopy show that 2-K(crypt) and 2-K(18c6) have t_1/2 values of 20 and 15 h at room temperature, respectively, vs. 1.5 h for 1. Complex 2-K(18c6) reacts with H₂ or PhSiH₃ to form the uranium hydride, [K(18-crown-6)(THF)₂][Cp′′₃UH], 3. Complexes 1 and 2-K(18c6) both reduce cyclooctatetraene to form uranocene, (C₈H₈)₂U, as well as the U³⁺ byproducts [K(2.2.2-cryptand)][Cp′₄U], 4, and Cp′′₃U, respectively.

A chromium(II)-based metal–organic framework Cr₃[(Cr₄Cl)₃(BTT)₈]₂ (Cr-BTT; BTT³⁻=1,3,5-benzenetristetrazolate), featuring coordinatively unsaturated, redox-active Cr²⁺ cation sites, was synthesized and investigated for potential applications in H₂ storage and O₂ production. Low-pressure H₂ adsorption and neutron powder diffraction experiments reveal moderately strong Cr–H₂ interactions, in line with results from previously reported M-BTT frameworks. Notably, gas adsorption measurements also reveal excellent O₂/N₂ selectivity with substantial O₂ reversibility at room temperature, based on selective electron transfer to form Cr^III superoxide moieties. Infrared spectroscopy and powder neutron diffraction experiments were used to confirm this mechanism of selective O₂ binding.

A chromium(II)-based metal–organic framework Cr₃[(Cr₄Cl)₃(BTT)₈]₂ (Cr-BTT; BTT³⁻=1,3,5-benzenetristetrazolate), featuring coordinatively unsaturated, redox-active Cr²⁺ cation sites, was synthesized and investigated for potential applications in H₂ storage and O₂ production. Low-pressure H₂ adsorption and neutron powder diffraction experiments reveal moderately strong Cr–H₂ interactions, in line with results from previously reported M-BTT frameworks. Notably, gas adsorption measurements also reveal excellent O₂/N₂ selectivity with substantial O₂ reversibility at room temperature, based on selective electron transfer to form Cr^III superoxide moieties. Infrared spectroscopy and powder neutron diffraction experiments were used to confirm this mechanism of selective O₂ binding.

A variety of strategies have been developed to adsorb and separate light hydrocarbons in metal–organic frameworks. Here, we present a new approach in which the pores of a framework are lined with four different C3 sidechains that feature various degrees of branching and saturation. These pendant groups, which essentially mimic a low-density solvent with restricted degrees of freedom, offer tunable control of dispersive host–guest interactions. The performance of a series of frameworks of the type Zn₂(fu-bdc)₂(dabco) (fu-bdc²⁻=functionalized 1,4-benzenedicarboxylate; dabco=1,4-diazabicyclo[2.2.2]octane), which feature a pillared layer structure, were investigated for the adsorption and separation of methane, ethane, ethylene, and acetylene. The four frameworks exhibit low methane uptake, whereas C2 hydrocarbon uptake is substantially higher as a result of the enhanced interaction of these molecules with the ligand sidechains. Most significantly, the adsorption quantities and selectivity were found to depend strongly upon the type of sidechains attached to the framework scaffold.

Single-molecule magnets comprising one spin center represent a fundamental size limit for spin-based information storage. Such an application hinges upon the realization of molecules possessing substantial barriers to spin inversion. Axially symmetric complexes of lanthanides hold the most promise for this due to their inherently high magnetic anisotropies and low tunneling probabilities. Herein, we demonstrate that strikingly large spin reversal barriers of 216 and 331 cm⁻¹ can also be realized in low-symmetry lanthanide tetraphenylborate complexes of the type [Cp*₂Ln(BPh₄)] (Cp*=pentamethylcyclopentadienyl; Ln=Tb (1) and Dy (2)). The dysprosium congener showed hysteretic magnetization data up to 5.3 K. Further studies of the magnetic relaxation processes of 1 and 2 under applied dc fields and upon dilution within a matrix of [Cp*₂Y(BPh₄)] revealed considerable suppression of the tunneling pathway, emphasizing the strong influence of dipolar interactions on the low-temperature magnetization dynamics in these systems.

The reaction of CuCl₂·2H₂O with three novel ditopic ligands, 2-methyl-1,4-benzeneditetrazolate (MeBDT^2–), 4,4′-biphenylditetrazolate (BPDT^2–), and 2,3,5,6-tetrafluoro-1,4-benzeneditriazolate (TFBDTri^2–), affords the metal–organic frameworks Cu(MeBDT)(dmf) (1), Cu(BPDT)(dmf) (2), and Cu(TFBDTri)(dmf) (3), respectively. These materials feature a common network topology in which octahedral Cu²⁺ ions are bridged by azolate ligands and dmf molecules to form one-dimensional chains. The individual chains are connected by the organic bridging units to form diamond-shaped channels, in which the solvent molecules project into the pores. The bridging dmf molecules in 1 are readily displaced by other coordinating solvent molecules, which leads to a change in the pore dimensions according to the steric bulk of the solvent. Interestingly, attempts to exchange the analogous solvent molecules in the expanded framework 2 induced no change in the pore size, revealing the rigidity of the framework. Meanwhile, 3 exhibits modest flexibility and an improved thermal stability consistent with its chemical functionality. The marked difference in flexibility highlights the considerable impact the organic linker can have on the dynamic framework properties.

Use of the ditopic ligand 1,4-benzenedi(1H-1,2,3-triazole) (H₂BDTri) enabled isolation of two new three-dimensional metal–organic frameworks of formulae Cu(BDTri)L in which L=DMF (1) and diethylformamide (DEF; 2). These compounds have the same primary structure, featuring one-dimensional channels with the bridging DMF or DEF molecules pointing into the cavity. Upon exposure to solvent vapors, both display a reversible flexibility, as characterized by single-crystal to single-crystal phase transitions in 1. The O₂ adsorption isotherms for the compounds show a two-step adsorption behavior associated with a permanent microporosity and a pore-opening process. In the case of N₂ adsorption, only 1 exhibits a two-step adsorption isotherm, whereas 2 does not present any pore opening, demonstrating that design of a flexible framework cavity can control the pore opening and thereby possibly enhance O₂/N₂ separation.

The escalating level of atmospheric carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO₂ emissions; however, currently the capture alone will increase the energy requirements of a plant by 25–40 %. This Review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO₂ emissions to the atmosphere, namely postcombustion (predominantly CO₂/N₂ separation), precombustion (CO₂/H₂) capture, and natural gas sweetening (CO₂/CH₄). The key factor which underlies significant advancements lies in improved materials that perform the separations. In this regard, the most recent developments and emerging concepts in CO₂ separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal–organic frameworks.

Metall-organische Gerüste sind wegen ihrer hohen Aufnahmekapazität bei tiefer Temperatur und der ausgezeichneten Reversibilitätskinetik interessant als mögliche Feststoffmaterialien zur Wasserstoffspeicherung. In den vergangenen Jahren hat man mehrere Methoden erforscht, um die Affinität dieser Materialien für Wasserstoff zu erhöhen, und die Bindung von H₂ an ungesättigte Metallzentren ist eine der vielversprechendsten. Wir geben hier eine Übersicht über bisher entwickelte Synthesemethoden für Gerüste mit zugänglichen Metallzentren sowie über die entsprechenden Aufnahmekapazitäten für Wasserstoff und Bindungsenergien. Weiterhin werden die Ergebnisse von Untersuchungen zur Metall-Wasserstoff-Wechselwirkung in ausgewählten Materialien diskutiert.

Use of the tetrahedral ligand tetrakis(4-tetrazolylphenyl)methane enabled isolation of two three-dimensional metal–organic frameworks featuring 4,6- and 4,8-connected nets related to the structures of garnet and fluorite with the formulae Mn₆(ttpm)₃⋅5 DMF⋅3 H₂O (1) and Cu[(Cu₄Cl)(ttpm)₂]₂⋅CuCl₂⋅5 DMF⋅11 H₂O (2) (H₄ttpm=tetrakis(4-tetrazolylphenyl)methane). The fluorite-type solid 2 displays an unprecedented post-synthetic transformation in which the negative charge of the framework is reduced by extraction of copper(II) chloride. Desolvation of this compound generates Cu₄(ttpm)₂⋅0.7 CuCl₂ (2 d), a microporous material exhibiting a high surface area and significant hydrogen uptake.

Owing to their high uptake capacity at low temperature and excellent reversibility kinetics, metal–organic frameworks have attracted considerable attention as potential solid-state hydrogen storage materials. In the last few years, researchers have also identified several strategies for increasing the affinity of these materials towards hydrogen, among which the binding of H₂ to unsaturated metal centers is one of the most promising. Herein, we review the synthetic approaches employed thus far for producing frameworks with exposed metal sites, and summarize the hydrogen uptake capacities and binding energies in these materials. In addition, results from experiments that were used to probe independently the metal–hydrogen interaction in selected materials will be discussed.